Heterogeneous networks comprise a variety of base stations serving mobile stations. In some systems, the base stations operate on the same carrier frequency. The variety of base stations can include some or all of the following types of base stations: conventional macro base stations (macro cells), pico base station (pico cells), relay nodes and femto base stations (also referred to as femto cells, closed subscriber group (CSG) cells or Home eNodeBs). Macro cells typically have coverage areas that range from several hundreds of meters to several kilometers. Pico cells, relays and femto cells can have coverage areas that are considerably smaller than the coverage area of typical macro cells. Pico cells can have coverage areas of about 100-200 meters. Femto cells are typically used for indoor coverage, and can have coverage areas in the 10 s of meters. Relay nodes are characterized by a wireless backhaul to a donor base station, and can have coverage areas similar to pico cells.
Heterogeneous networks can potentially enable an operator to provide improved service to users (e.g., increased data rates, faster access, etc.) with lower capital expenditure. Typically, installation of macro base stations is very expensive as they require towers. On the other hand, base stations with smaller coverage areas are generally much less expensive to install. For example, pico cells can be installed on roof tops and femto cells can be easily installed indoors. The pico and femto cells allow the network to offload user communication traffic from the macro cell to the pico or femto cells. This enables users to get higher throughput and better service without the network operator installing additional macro base stations or provisioning more carrier frequencies for communication. Thus, heterogeneous networks are considered to be an attractive path for evolution of wireless communication networks. 3GPP has commenced work on enabling heterogeneous LTE networks in 3GPP LTE Release 10.
FIG. 1 illustrates an LTE Heterogeneous network comprising a macro cell, pico cells and femto cells operating on a single carrier frequency. A mobile station, also referred to as “user equipment” (UE), may be associated with one of the cells based on its location. The association of a UE to a cell can refer to association in idle mode or connected mode. That is, a UE is considered to be associated with a cell in idle mode if it is camped on the cell in idle mode. Similarly, a UE is considered to be associated with a cell in connected mode if it is configured to perform bi-directional communication with a cell (for example, a UE in LTE radio resource control (RRC) connected mode can be connected to and therefore associate with a cell). A UE associated with a macro cell is referred to macro UE, a UE associated with a pico cell is referred to as a pico UE, and a UE associated with a femto cell is referred to as a femto UE.
Various time-division approaches are possible for ensuring that base stations in heterogeneous networks share the frequency spectrum while minimizing interference. Two approaches can be envisioned. A network can configure time periods when different base stations are required to not transmit. This enables cells that can interfere with one another to transmit in mutually exclusive time periods. For example, a femto cell can be configured with some time periods during which it does not transmit. If a macro UE is located within the coverage of the femto cell, the macro cell can use the time periods during which the femto cell does not transmit data to the UE.
The network can configure time periods where a first base station transmits on all available time periods (e.g., pico eNBs), while a second base station (e.g., macro eNB) transmits only on a subset of the available time periods. A UE connected to the first base station can therefore have two “virtual” channels at different channel qualities depending on how much the second base station's transmission interferes with that for the first (i.e., signal geometry of the first base station relative to the second). The first virtual channel is where only the first base station transmits data while the second base station does not transmit data. The second virtual channel is one where both the first and the second base stations transmit data. The first base station can use adaptive modulation and coding and schedule at different modulation coding scheme (MCS) levels on two virtual channels. In the extreme case, the first base station may not schedule at all on the second virtual channel when interference from the second base station is large.
However, it should be noted that time division approaches can lead to inaccurate or inconsistent radio resource management (RRM) measurements, RLM measurements and channel state information (CSI) measurements. For example, if a macro UE located near a femto cell performs measurements during time periods when the femto cell transmits, the measured values can be significantly different from measured values obtained from measurements made during time periods when the femto cell does not transmit. Such measurements can lead to erratic behaviors, such as failed connections, unnecessary handovers and unnecessary cell reselections. Therefore, methods are needed for performing measurements of cells that overcome the problems mentioned above.
The various aspects, features and advantages of the disclosure will become more fully apparent to those having ordinary skill in the art upon a careful consideration of the following Detailed Description thereof with the accompanying drawings described below. The drawings may have been simplified for clarity and are not necessarily drawn to scale.